Found within the technical field of high concentration solar thermoelectricity are solar central tower systems in which direct solar radiation is reflected by a field of heliostats towards the receiver, situated on the upper part of the tower (optical system focus). The receiver is the system where all the solar radiation coming from the solar field concentrates. This solar energy transforms into the thermal energy of the heat transfer fluid. In the event that water is not used as a heat transfer fluid, a heat exchanger is required where the thermal energy from the heat transfer fluid is transferred to the water for the production of steam, overheated at a high temperature. Finally, this overheated steam is carried to the turbine and then to an alternator to generate electricity.
Currently, there are different types of receivers to which variations can be made to the heat transfer fluid as well as the way the energy is transferred to it or the configuration of the receiver itself. Of this type, there are pipe receivers, volumetric receivers, receivers of direct or indirect energy exchange, saturated steam and overheated steam receivers, etc.
The receivers on concentrated solar power towers can be exterior or dispose of a cavity located on the upper part of said tower with the aim of reducing thermal losses. The configuration must allow the incident power to exceed the losses caused by radiation and convection. In molten salt receivers, the temperature reached on the surface of the receiver is greater than in saturated and overheated water/steam receivers, which is why radiation losses are also greater; however, this is solved by generating overheated steam at higher temperatures through the use of the exchanger, which leads to greater efficiency in the thermodynamic cycle.
Therefore, the main advantage of molten salt receiver plants stems from the fact that, to work with heat transfer fluid at high-energy levels, after the use of the heat exchanger for the production of overheated steam, temperatures are reached that are greater (550° C.) than those obtained in plants with receivers that directly produce overheated steam (520° C.); thus the efficiency of the thermodynamic cycle is increased and consequently increases the performance of the turbine. If instead of using water/steam, we use a mixture of molten salts as a heat transfer fluid, it is estimated that the efficiency of the cycle can increase from 28% to 38%.
The salt towers hitherto existent have presented technical and economic difficulties that have led to short operating periods, which is not viable for a commercial plant in which life spans of between 20 years and 25 years are demanded.
Patent US2008000231 is a clear example of the technology that has existed up until now. Within this patent there is a description of a central tower solar receiver system using molten salts, whose panels are supplied with the salt originating from a cold tank. Once heated, the heat transfer fluid transfers its heat to an exchanger for the production of steam, which is finally used for the production of electricity. The heat transfer fluid which is already cold is sent back to the receiver so that it can be heated again.
The technical problems presented during operation in these plants are mainly related to the resistance of the materials and to the control of the system during transitory states (passing of clouds). Some of the technical difficulties encountered were the appearance of cracks in the sections of the pipes welded to the collectors, where the entrance and exit of the working fluids (headers) take place. This is due to high thermal gradients, the corrosion of the valves for flow distribution control caused by the highly corrosive effect of the mixture of molten salts at high working temperatures, the solidification of the heat transfer fluid inside the pipes especially in areas with a low flow of incident radiation, etc.
The configurations of salt receivers hitherto existing are cylindrical and external. A receiver of this type is composed of panels in groups of eight panels, so that the receiver is divided into four sections (north-east, northwest, south-west and south-east). In turn, each panel is composed of thirty-two vertical pipes.
The entrance of cold heat transfer fluid (cold salts), originating from the cold storage tank, takes place in the central panels situated at the north face (first panel north-east section and first panel north-west section). Half of the entering flow is carried towards the panels in the north-east section and the other half towards the north-west section, with a descending vertical path, alternatively ascending in the adjacent panels (path in the shape of a serpent). Once the eight panels situated at the north face (north-east and north-west) have been passed through, a cross flow is produced, which is to say, the exiting flow from the north-east panels enters the section with the south-west panels and the exiting flow from the north-west panels enters the section with the south-east panels.
As with the sections situated at the north face, the flow of the heat transfer fluid inside the pipes of the adjacent panels is serpent-shaped until it exits.
The exit of the heat transfer fluid takes place at the central panels situated at the south face of the receiver.
According to that explained previously, the molten salt receivers can present various difficulties such as for example, large thermal losses if dealing with an external receiver and structural damage due to the high operating temperatures, the distribution of incident flow (not uniform), thermal tensions to which the material is subjected and the effects of corrosion on the material.
The thermal cycles are generated due to the exposure of the receiver surface to concentrated solar radiation by the heliostat field (with which the metal in the pipes reaches temperatures close to 800° C.), and to the heat transfer fluid temperature gradient between the entrance (290° C., melting point) and the exit of the receiver (565° C., degradation temperature of the salt).
The aforementioned problems with molten salt receivers can be reduced through the use of this new receiver design when dealing with a cavity receiver (installed in the high part of a tower inside a gap or cavity) which also reduces the temperature gradient between the exit and the entrance of the molten salts in the receiver thanks to the recirculation which is explained below. To do so, this invention aims to recirculate a part of the exit flow (mixture of hot molten salts).
The importance of the design and the configuration of the receiver stems from here. A suitable design and configuration of the receiver will lead to stable system control during the operation of the solar plant, especially during transitory periods (passing of clouds); guaranteeing in this way the integrity of the structure and its durability (useful life of the receiver between 20 and 30 years).
Furthermore, the use of molten salts as a heat transfer fluid and a means of thermal storage allows improvements in the efficiency of the thermoelectric solar plant, given that the temperatures reached are higher without the need to increase the operating pressure as would be required with water/steam; which implies a lower cost of the receiver.